This project is conccerned with the mechanism of action of thymidylate synthetase (deoxyuridylate plus 5,10-methylenetetrahydrofolate yields thymidylate plus 7,8-dihydrofolate) from amethopterin-resistant Lactobacillus casei. The problems to be investigated include (a) characterization of pseudo ternary complexes of thymidylate synthetase, (b) equilibrium gel filtration studies of binary enzyme complexes, (c) active site crosslinking studies, (d) modification of carboxyl groups with water-soluble carbodiimides, (e) characterization of enzyme-Cibacron Blue interactions.